The present disclosure relates generally to downhole radiation generation for nuclear well logging and, more particularly, to electrode configurations for downhole nuclear radiation generator tubes.
A downhole generator tube may include three main components: an ion source, an acceleration column, and a target. An ion beam from the ion source may advance through the acceleration column toward the target, guided by a potential difference between an electrode near the ion source and an electrode near the target. Neutrons and/or gamma-rays are generated when the accelerated ions strike the target. As the ion beam progresses through the acceleration column, however, some of the ions may strike an electrode instead of the target. This may occur in part because the acceleration column of a downhole neutron generator tube may hold a pressurized gas, rather than a vacuum, and ions from the ion beam may strike pressurized gas particles in the acceleration column and change direction.
When an ion from the ion beam impinges on an electrode in the acceleration column, ion-induced sputtering may result. Sputtering causes the emission and transport of electrode material, which generally may be isotropic and generally may travel in a straight line from the point of emission. As a result, electrically conductive electrode material may condense on nearby ceramic high voltage insulators that surround the acceleration column. If the high voltage insulators are coated by sputtered electrode material across a substantial length of the acceleration column, the voltage potential between the electrode near the ion source and the electrode near the target may short circuit in a catastrophic leakage event. Even if the acceleration column does not short circuit, sputtered electrode material along the high voltage insulator may form a conductive deposited film that takes on an intermediate voltage between the potential of the ion source and the potential of the target. This conductive film may increase electric field stresses on the adjacent electrodes in the acceleration column. Increased electrical field stresses may yield an increase in a high voltage leakage current, as well as increase the likelihood of catastrophic leakage events due to leakage currents on the insulator or field emission from one of the electrodes.
Uneven target surface wear may also be problematic for a downhole neutron generator. Because the ion beam from the ion source to the target may be center-weighted, the ion beam may be unevenly distributed across the beam spot upon striking the target. This uneven distribution may generate uneven wear on the end of the target, which may cause the neutron yield of the neutron generator to diminish as part of the target wears out prematurely.
Similarly, a downhole x-ray generator tube also may include three main components: an electron emitter (cathode), an acceleration column, and a target (anode). An electron beam from the cathode may advance through the acceleration column toward the anode, guided by the potential difference between an electrode near the electron gun (cathode) and the anode or an adjacent electrode. X-rays are generated through Bremsstrahlung or characteristic x-ray emission following inner shell ionization when the electrons hit the anode and are decelerated and scattered in the material. As the electron beam progresses through the acceleration column, however, some of the electrons may strike an electrode instead of reaching the anode. For this reason and others, many of the same problems mentioned above may affect downhole neutron generator tubes as well as x-ray generator tubes.